Scissors-type hoist mechanism

ABSTRACT

A hoist for use in sheet piling operations or the like which comprises a base frame and a vertically movable platform supported thereon. The platform is raised and lowered by operation of a single power driven lift screw mounted in an upright end portion of the base frame with the platform being connected to a bottom, horizontally disposed portion of the base frame by a scissors mechanism which holds the platform in a plane parallel with the plane of the horizontal portion of the base frame as the platform is raised and lowered.

United States Patent n91 Buccicone 51 Apr. 17,1973

[73] Assignee: Bucciconi Engineering Co., Inc.,

Gary, Ind.

[22] Filed: Jan. 4, 1971 [21] Appl.No.: 103,709

[52] US. Cl. ..2l4/95 R, 214/6 H 2,306,101 12/1942 Zandt ..193/35 A 2,052,119 8/1936 Tear l87/8.52 2,843,222 7/1958 Villars 1. 187/24 Primary Examiner-Robert G. Sheridan Assistant Examinerl .awrence J. Oresky AttorneyGuy A. Greenawalt ABSTRACT A hoist for use in' sheet piling operations or the like [51]- Int. Cl ..B65g 47/00 which comprises a base frame and a vertically mova- [58] Field of Search 14/6 H, 95; ble platform supported thereon. The platform is raised 193/35 A, 35 SS; 198/34; 187/24 and lowered by operation of a single power driven lift screw mounted in an upright end portion of the base [56] References Cited frame with the platform being connected to a bottom, horizontally disposed portion of the base frame by a UNITED STATES PATENTS scissors mechanism which holds the platform in a 3,369,675 2/1968 Buccicone ..254/l26 plane parallel with the Plane of the horizontal P F 1,101,598 6/1914 187/24 of the base frame as the platform is raised and 2,924,323 2/1960 198/34 lowered. 2,527,897 10/1950 ...187/25 3,394,792 7/1968 Aruidson et a.l. 198/24 8 Claim, 9 Drawing Figures PATENTED APR 1 71975 SHEET 2 [IF 5 INVENTOFI VEL/O S. BUCCICONE PATENTEDAPRWW 3,727. 780

SHEET 3 UF 5 INVENTOR VEL/O 51 BUCC/CONE ATT'YS.

PATENTED 1 3. 727. 780

sum u or 5 Illlll INVENTOR l/fL/O 5. BUCCICONE ATT'Y PATENTEB APR 1 71975 SHEET 5 0F 5 fV/A INVENTOR VEL/O 5. BUCC/CONE QMWMQ ATT'YS.

SCISSORS-TYPE I-IOIST MECHANISM This invention relates to apparatus for handling sheet materials and is more particularly concerned with improvements in a lift or hoist structure which is especially adapted for use in sheet piling operations for supporting a pile of metal sheets which are deposited thereon.

In the handling of metal sheets, apparatus has been developed for piling the sheets as they are advanced from an oscillating shear or the like by overhead magnetic conveyors which are operated to release the sheets for deposit in one or more piles with provision for stopping the advance of the sheets and guiding the same downwardly into a pileon a hoist or similar supporting structure. In some piling operations, it is desirable to provide a hoist arrangement on which the sheets can be piled or stacked with provision for adjusting the elevation of the top of the'stack so that the distance a sheet must fall before coming to rest on the top of the stack may be maintained fairly constant by gradual lowering of the stack as the successive sheets are deposited thereon. Various types of hoist equipment have been used in prior piling machines. One such machine which employs a scissors-type hoist, is shown in Buccicone U.S. Pat. No. 3,369,675, dated Feb. 20, 1968. Experience with this machine has indicated that in some operations a simpler hoist construction would be advantageous particularly in the handling of sheets of lesser weight and/or size than those for which the hoist equipment in U.S. Pat. No. 3,369,675 has been.

designed. Accordingly, it is a general object of the present invention to provide a hoist construction suitable for use with a sheet piling machine which is relatively simple in design, which can be produced economically, and which will operate with a high degree of efficiency in handling sheets of relatively light weight and/or relatively small size.

It is a more specific object of the invention to provide a scissors-type hoist which is especially adapted for use in sheet piling or sheet stacking operations, which is of relatively light construction, which is easy to operate, which is economical to build and which has a high degree of stability when used for stacking relatively small, lightweight sheets.

It is a further object of the invention to provide a scissors-type hoist having a stack receiving frame which is mounted on a main support frame for vertical movement by means of a single lift screw and associated scissors-type arms, with the lift screw being mounted in an upright frame unit which forms a back stop or aligning surface for stacking sheets against it and with provisions for holding the stack against unintentional discharge by driven rollers on which the sheets are supported and which enable the stack, when desired, to be moved in a lateral direction from the hoist for transfer to another supporting means.

These and other objects and advantages of the invention will be apparent from a consideration of the hoist apparatus which is shown by way of illustration in the accompanying drawings wherein:

FIG. 1 is a side elevation of a hoist or lift apparatus which is especially adapted for use in connection with the stacking of metal sheets and which embodies therein the principal features of the invention;

FIG. 2 is an elevational view showing the end or side of the apparatus at which a power driven hoist screw and associated mechanism is mounted;

FIG. 3 is a plan view of the apparatus with portions broken away;

FIG. 4 is a partial vertical cross section, taken on the.

line 44 of FIG. 2, with portions broken away, and to an enlarged scale, the view showing the lift screw and associated mechanism;

FIG. 5 is a partial vertical sectional view, taken on the line 5-5 of FIG. 2, with portions broken away and to an enlarged scale;

FIG. 6 is a fragmentary sectional view taken on the line 66 of FIG. 2, to an enlarged scale and with portions broken away;

FIG. 7 is a fragmentary sectional view taken on the line 7-7 of FIG. 3, to an enlarged scale;

FIG. 8 is a fragmentary sectional view taken on the line 88 of FIG. 4, to an enlarged scale;

FIG. 9 is a fragmentary sectional view taken on the line 9-9 of FIG. ,1, to an enlarged scale.

Referring to FIG. I particularly, the illustrated hoist structure comprises a base or support frame 10 having a horizontally disposed main frame portion 12, of rectangular shape, and an upright frame portion 14 at one end in which a vertically disposed screw 16 is mounted for raising and lowering, by means of a connecting bracket and nut, a platform assembly 18 on which a plurality of sheets are adapted to be piled. The base frame 10 and the platform assembly 18 are vertically aligned and connected by a pair of laterally spaced, scissors-like arm assemblies 20 which collapse or fold when the platform assembly 18 is moved to its lowermost position on the base frame 10.

The pile supporting platform or sheet supporting table assembly 18 includes a main frame 21 of rectangular shape corresponding approximately to the shape of the horizontally disposed portion 12 of the base frame structure 10 which frame 12 is mounted in verti cally movable relation above the base frame portion 12. The platform frame 21 includes a base or bottom plate 22 on the bottom face of which there are depending side plates 23 24 (FIG. 2) and dependent end plates 25, 26 (FIGS. 1 and 2). Apair of angle members 27, 28 are mounted along the margins of the base plate 22 which form parallel, spaced bearing supports for a plurality of parallel pile supporting rollers 30. The rollers 30 are arranged in parallel, closely spaced relation so as to provide a supporting table for the sheets. Al-

ternately spaced rollers 30 carry at their outer ends sprockets 31 and 32 which are connected in driving relation by chains 33. Sprockets 34 are provided on certain of the rollers indicated at 30', which are driven by the chains 33 connecting the sprockets 31 32 on the adjacent rollers. The sprocket 32 on the one endmost roller 30 is connected in driving relation by a chain 35 (FIGS. 6 and 7) with a sprocket 36 on the end of a drive shaft 37. The drive shaft 37 is mounted in bearings 38, 38' at one end or side of the platform main frame 21. The drive shaft 37 carries at its opposite end a gear 40 which engages with gear 41 on the output shaft 42 of a drive motor 43. The drive motor 43 is supported on a bracket or platform extension 44 at the end of the platform assembly 18, the bracket 44 being attached to an extended portion45 of the base plate 22 which is notched out at 46 to accommodate the gear 41. The gear and chain drive at the free end of the plat form assembly 18 is provided with a cover member 47 while a cover member or guard 48 is provided for the gears 40 and 41.

At one side of the platform assembly 18 a guard or stop plate 50 (FIGS. 2 and 7) is disposed between the two endmost rollers 30, 30 which plate member 50 is pivotally supported at its opposite ends by an angle bracket 51 (FIG. 1) and the roller supporting angle member 28. The plate 50 is of generally rectangular shape and is pivotally supported-offcenter so that it is normally biased by its own weight into a vertical plane where the top margin acts as a stop for the bottom sheets in a pile which is accumulated on the top of rollers 30, 30 and provision is made for automatically swinging the plate 50 out of stop forming position when the platform 18 is dropped to its lowermost position as hereinafter described. The stop plate 50 may, of course, be of different cross sectional configuration and weighted or counterbalanced to normally swing into the desired position in a vertical plane when the platform 18 is raised.

The main base 'or supporting frame comprises a bottom or base plate 55 and upstanding side plates 56, 57 with a cross plate 58 at the end opposite the upright frame portion 14. The end frame 14 in which the vertical screw 16 is mounted comprises a vertically disposed base or back plate 60 which is set inwardly of the end or edge of the bottom frame plate 55 as shown in FIGS. 1 and 4. Two parallel, laterally spaced plates 61 and 62 (FIGS. 1 to 4) are mounted on the outer face of the vertical frame plate 60 which form with a center portion of the back plate 60 an outwardly opening housing for the screw 16 which extends upwardly from a right angle gear assembly 63 (FIGS. 2 and 4). The gear drive assembly 63 is mounted on the bottom frame plate 55 with the top end of the screw 16 being supported in a bearing assembly 64 mounted in a top plate 65 which extends from the outer face of the vertical back plate 60 parallel with the marginal portion of the bottom plate 55. A hoist nut 66 is carried in threaded relation on the vertically disposed screw 16 and a nut housing 67' encompasses the same. The housing 67 is secured thereon by a nut 68 on an exteriorly threaded extension on the top of the hoist nut 66. The nut housing 67 has cylindrical bearing members 70, 70 (FIGS. 1, 3, 4 and 8) extending from the exterior surfaces ofbppositely disposed side walls for receiving thereon parallel arms or plate members 72, 72' of a bracket assembly 73 which extends from the side plate 38' of the main frame 21 of the platform assembly 18 and serves to connect the latter for vertical movement with the hoist nut 66. A guide roller assembly 75 is secured on the outer ends of the hook-shaped plate members 72, 72' of the bracket 73. The guide roller assembly 75, which also serves to relieve the tilting stress on the nut 66, comprises a generally U-shaped frame member 76 having a bight forming plate portion 76' (FIG. 4) attached to a mounting plate 77 which is in turn attached to the ends of the plate members 72, 72. The parallel legs 78, 78' of frame member 76 extend outwardly in generally parallel relation adjacent the frame plates 61 and 62 and are connected by a cross brace or reinforcing plate 79. Each of the legs 78, 78 has a section of angle bar 80, 80' mounted on the oppositely disposed, outside margins so that the one leg or flange thereof will cooperate with the marginal portions of the legs 78, 78' in providing bearings for a plurality of anti-friction rollers 81, 81' which are adapted to ride on the outside edges 61' and 62 of the parallel, vertical frame plates 61 and 62.

A drive motor 82 (FIGS. 1 to 3) is mounted by means of bracket 83 on the back plate 60 with an output shaft 84 carrying a pulley 85 which is connected by a drive belt 86 with a pulley 87 on the drive shaft 88.

The drive shaft 88 is connected by coupling 88' to the input shaft 89 for the right angle gear box 63 which has an associated brake assembly 90. The drive shaft 88 is mounted in suitable bearings 91 on the bottom plate 55.

The one vertically disposed frame plate 61 is slotted at 92 and 93 (FIG. 5) and limit switches 94 and 95 are mounted thereon with their operating arms extending through the slots 92 and 93 in the path of an operating shoe 96 carried on the side wall of the nut housing 67. The limit switches 94 and 95 are connected into the electrical control circuit for the drive motor 82 so as to limit the movement of the platform assembly 18.

The scissors arm assemblies 20 (FIGS. 1, 3 and 9) which connect the platform assembly 18 and the base frame 10 are identical and each of these assemblies comprises two arm members 100, 101 which are connected intermediate their .ends by a pivot 102. Each of the arm members 100, 101 is fabricated of two plate or bar members 103, 103' arranged in parallel relation and separated by suitable spacers or connectors which .permit interleaving when the arm members are pivoted to a collapsed position. The one arm member is pivotally connected at one end by pivot 104 to a bracket 105 secured on the bottom frame 10 at the base of the end frame 14. A roller member 106 is mounted on a pivot 107 at the other end of the arm 100 which has a grooved periphery 108 for riding on a track 110 on a bearing plate 110 which is secured on the bottom face of the platform plate 22. The associated arm 101 is connected at the one end by pivot 111 to a bracket 112 secured on the bottom face of platform plate 22 adjacent the side frame member 38. A roller member 113 is mounted on a pivot 114 at the other end of arm 101 which rides on a track plate 115 mounted on the bottom frame plate 55, the construction of roller member 113 and bearing or track plate member 115 being the same as roller member 106 and track plate member 110. The two arm assemblies illustrated are spaced laterally of the base frame 10 and the platform assemblies 18 as shown and co-operate in supporting and maintaining the platform assembly 18 in a level position as it is raised and lowered by operation of the lift screw 16.

The base or bottom frame 10 has a cam member and supporting bracket 121 (FIGS. 1 and 7) mounted on the side plate 56 which cam member 120 is in the path ofa co-operating cam member 122 on the pivoted guard or stop rail 50 so as to swing the pile stop member 50 to an inoperative position as shown in solid line in FIG. 7 when the platform assembly is dropped to its lowermost position on the bottom frame 10. The platform plate member 22 is aperturedat 123 to accommodate the cam members 120 and 122.

The vertical frame plate 60 is provided with a bumper stop forming plate member 125 (FIG. 1) for engagement by the leading edges of the sheets when the hoist is used in a sheet piling operation where the sheets are deposited thereon by an overhead conveyor.

The use of the'hoist or lift apparatus will be apparent from the foregoing description. The platform assembly 18 is raised or lowered as desired by operation of the lift screw 16. The limit switches 94 and 95 serve as safety devices to prevent damage to the apparatus. The driven rollers 30, 30 permit ready transfer of the pile from the apparatus when the platform assembly 18 is lowered onto the base frame 10. The drive motors 43 and 82 may, of course, be operated through a suitable electric circuit controlling the current supplied to the same.

I claim:

I. A scissors-type hoist mechanism comprising a generally rectangular base support frame adapted to be positioned in a generally horizontal plane, an upright end support frame at one side of said base support frame, a single power driven, vertically disposed, lift screw mounted in said end support frame, a sheet supporting platform assembly disposed for movement in a vertical path above said base support frame, a nut carried on said lift screw which is driven in a vertical path by rotation of said lift screw, a housing encompassing said nut, said nut housing having laterally extending bearing forming means on oppositely disposed walls, a hook-shaped bracket on said platform assembly which extends at the side adjoining said lift screw, said bracket having a downwardly opening recessed bottom edge adapted to seat on said bearing forming means so as to detachably connect said bracket to said nut housing for movement therewith, a pair of scissors-like arm assemblies connecting said platform assembly and said base support frame for holding said platform assembly in a plane generally parallel with the plane of said base support frame during movement of said platform assembly, said platform assembly having a plurality of sheet supporting rollers thereon and power means carried on said platform assembly for driving said rollers so as to enable sheets thereon to be discharged from said platform assembly.

2. A scissors-type hoist mechanism adapted for use in supporting a pile of metal sheets which are dropped thereon by an overhead conveyor in a sheet piling machine, said hoist mechanism comprising a generally rectangular base support frame, an upright end support frame at one side of said base support frame, a single power driven, vertically disposed, lift screw mounted in said end support frame, a sheet supporting platform assembly disposed for vertical reciprocating movement above said base support frame, a lift nut carried on said lift screw which is driven by rotation of said lift screw, a hook-shaped bracket on said platform assembly which extends at the side adjoining said lift screw and is detachably connected to said lift nut for movement in a vertical path, said lift nut having a housing secured thereon with laterally extending bearing members, said hook-shaped bracket comprising parallel, spaced, vertically disposed plate members with recesses in their lower edges in which said bearing forming members are seated, a pair of scissors-like arm assemblies connecting said platform assembly and said base support frame for holding said platform assembly in a plane generally parallel with the plane of said base support frame during vertical movement of said platform assembly, said platform assembly having a plurality of sheet supporting rollers thereon and power means for driving said rollers so as to enable sheets thereon to be discharged from said platform assembly in a direction laterally of said end support frame.

3. A scissors-type hoist mechanism as set forth in claim 1 wherein said upright support frame has vertically disposed truck-forming members adjacent opposite sides of said lift screw housing and said hookshaped bracket has a carriage forming frame mounted on the extended end thereof with laterally spaced antifriction means which are positioned to engage and ride on said track forming members so as to hold said platform against twisting movement as it traverses a vertical path.

4. A scissors-type hoist mechanism as set forth in claim 1 wherein said end support frame includes parallel plate members mounted on opposite sides of said lift screw and having parallel track-forming outer edges extending along said screw, and said hook-shaped bracket comprising parallel end plate members with hook forming outer ends which seat on said bearing forming means and carriage forming means on the ends of said plate members having anti-friction rollers disposed to ride on the track-forming edges of said frame plate members so as to relieve the stress on said lift nut and lift screw resulting from the weight of said platform assembly.

5. A scissors-type hoist mechanism as set forth in, claim 2 and said end support frame having a vertically disposed bumper pad mounted on the inside face thereof adjacent the top end so as to form. a stop member for engagement by the leading ends of sheets which have a forward momentum when released from a conveyor for deposit on said platform assembly.

6. A scissors-type hoist mechanism for supporting sheets in a pile thereon which mechanism comprises a generally rectangular, horizontally disposed base support frame, an upright end support frame at one side of said base support frame, a power driven, vertically disposed lift screw mounted in said end support frame, a sheet supporting platform assembly disposed for movement in a vertical path above said base support frame, a nut carried on said lift screw which is driven in a vertical path by rotation of said lift screw, a housing carried on said nut, said nut housing having laterally extending cylindrical bearing members on opposite faces thereof, a hook-shaped bracket on said platform as sembly which extends at the side adjoining said lift screw, said hook-shaped bracket having laterally spaced, parallel, side plate members which are recessed on the bottom to fit over said bearing members and detachably connect said bracket to said nut housing for vertical movement therewith, scissors-like arm assemblies disposed in spaced vertical planes and connecting said platform assembly and said base support frame for holding said platform assembly in a plane generally parallel with the plane of said base support frame during vertical movement of said platform assembly, said platform assembly having a plurality of sheet'supporting rollers mounted thereon and power means carried on said platform assembly for driving said rollers in a direction to discharge sheets thereon at one side of said platform assembly.

7. A scissors-type hoist mechanism as set forth in claim 6 wherein said platform assembly includes a stop member disposed at the discharge side of said platform for normally preventing removal of sheets thereon by rotation of said sheet supporting rollers, which stop member comprises an elongate bar member mounted disposed in the lowermost position.

8. A scissors-type hoist as set forth in claim 6 wherein said platform assembly is provided with a stop member comprising an elongate bar which is pivotally mounted at its opposite ends along the discharge side of said platform assembly and said stop bar is balanced on its pivot mounting so that it normally pivotsto a position where the top margin projects above the plane of the top face of said platform assembly, and wherein said base support frame has a cam forming member thereon which is disposed to engage said bar when said platform assembly is lowered to a position adjacent said base support frame and pivot the same from its normal operative position to an inoperative position where the top margin is below the plane of the top face of the said platform assembly. 

1. A scissors-type hoist mechanism comprising a generally rectangular base support frame adapted to be positioned in a generally horizontal plane, an upright end support frame at one side of said base support frame, a single power driven, vertically disposed, lift screw mounted in said end support frame, a sheet supporting platform assembly disposed for movement in a vertical path above said base support frame, a nut carried on said lifT screw which is driven in a vertical path by rotation of said lift screw, a housing encompassing said nut, said nut housing having laterally extending bearing forming means on oppositely disposed walls, a hook-shaped bracket on said platform assembly which extends at the side adjoining said lift screw, said bracket having a downwardly opening recessed bottom edge adapted to seat on said bearing forming means so as to detachably connect said bracket to said nut housing for movement therewith, a pair of scissors-like arm assemblies connecting said platform assembly and said base support frame for holding said platform assembly in a plane generally parallel with the plane of said base support frame during movement of said platform assembly, said platform assembly having a plurality of sheet supporting rollers thereon and power means carried on said platform assembly for driving said rollers so as to enable sheets thereon to be discharged from said platform assembly.
 2. A scissors-type hoist mechanism adapted for use in supporting a pile of metal sheets which are dropped thereon by an overhead conveyor in a sheet piling machine, said hoist mechanism comprising a generally rectangular base support frame, an upright end support frame at one side of said base support frame, a single power driven, vertically disposed, lift screw mounted in said end support frame, a sheet supporting platform assembly disposed for vertical reciprocating movement above said base support frame, a lift nut carried on said lift screw which is driven by rotation of said lift screw, a hook-shaped bracket on said platform assembly which extends at the side adjoining said lift screw and is detachably connected to said lift nut for movement in a vertical path, said lift nut having a housing secured thereon with laterally extending bearing members, said hook-shaped bracket comprising parallel, spaced, vertically disposed plate members with recesses in their lower edges in which said bearing forming members are seated, a pair of scissors-like arm assemblies connecting said platform assembly and said base support frame for holding said platform assembly in a plane generally parallel with the plane of said base support frame during vertical movement of said platform assembly, said platform assembly having a plurality of sheet supporting rollers thereon and power means for driving said rollers so as to enable sheets thereon to be discharged from said platform assembly in a direction laterally of said end support frame.
 3. A scissors-type hoist mechanism as set forth in claim 1 wherein said upright support frame has vertically disposed track-forming members adjacent opposite sides of said lift screw housing and said hook-shaped bracket has a carriage forming frame mounted on the extended end thereof with laterally spaced anti-friction means which are positioned to engage and ride on said track forming members so as to hold said platform against twisting movement as it traverses a vertical path.
 4. A scissors-type hoist mechanism as set forth in claim 1 wherein said end support frame includes parallel plate members mounted on opposite sides of said lift screw and having parallel track-forming outer edges extending along said screw, and said hook-shaped bracket comprising parallel end plate members with hook forming outer ends which seat on said bearing forming means and carriage forming means on the ends of said plate members having anti-friction rollers disposed to ride on the track-forming edges of said frame plate members so as to relieve the stress on said lift nut and lift screw resulting from the weight of said platform assembly.
 5. A scissors-type hoist mechanism as set forth in claim 2 and said end support frame having a vertically disposed bumper pad mounted on the inside face thereof adjacent the top end so as to form a stop member for engagement by the leading ends of sheets which have a forward momentum when released from a conveyor for deposit on said platform assembly.
 6. A scissoRs-type hoist mechanism for supporting sheets in a pile thereon which mechanism comprises a generally rectangular, horizontally disposed base support frame, an upright end support frame at one side of said base support frame, a power driven, vertically disposed lift screw mounted in said end support frame, a sheet supporting platform assembly disposed for movement in a vertical path above said base support frame, a nut carried on said lift screw which is driven in a vertical path by rotation of said lift screw, a housing carried on said nut, said nut housing having laterally extending cylindrical bearing members on opposite faces thereof, a hook-shaped bracket on said platform assembly which extends at the side adjoining said lift screw, said hook-shaped bracket having laterally spaced, parallel, side plate members which are recessed on the bottom to fit over said bearing members and detachably connect said bracket to said nut housing for vertical movement therewith, scissors-like arm assemblies disposed in spaced vertical planes and connecting said platform assembly and said base support frame for holding said platform assembly in a plane generally parallel with the plane of said base support frame during vertical movement of said platform assembly, said platform assembly having a plurality of sheet supporting rollers mounted thereon and power means carried on said platform assembly for driving said rollers in a direction to discharge sheets thereon at one side of said platform assembly.
 7. A scissors-type hoist mechanism as set forth in claim 6 wherein said platform assembly includes a stop member disposed at the discharge side of said platform for normally preventing removal of sheets thereon by rotation of said sheet supporting rollers, which stop member comprises an elongate bar member mounted for swinging movement on end pivots, means causing said bar member to swing to a position where the topmost edge normally extends above the plane of the sheet supporting top face of said platform assembly, and wherein a cam forming bracket is provided on said base support frame for biasing said bar member to an inoperative position where the topmost edge is ineffective as a stop member when said platform assembly is disposed in the lowermost position.
 8. A scissors-type hoist as set forth in claim 6 wherein said platform assembly is provided with a stop member comprising an elongate bar which is pivotally mounted at its opposite ends along the discharge side of said platform assembly and said stop bar is balanced on its pivot mounting so that it normally pivots to a position where the top margin projects above the plane of the top face of said platform assembly, and wherein said base support frame has a cam forming member thereon which is disposed to engage said bar when said platform assembly is lowered to a position adjacent said base support frame and pivot the same from its normal operative position to an inoperative position where the top margin is below the plane of the top face of the said platform assembly. 